U.S. Pat. No. 2,833,816 was the first to disclose the unique catalysis resulting from the combination of a source of bromine and a source of one or more transition, polyvalent metal oxidation catalysts for the liquid phase oxidation with a source of molecular oxygen of di-, tri- and higher alkyl-substituted aromatic compounds (e.g., xylenes, tri- and tetramethyl benzenes) to aromatic di-, tri- and higher carboxylic acids (e.g., phthalic acids and benzene tri- and tetracarboxylic acids). Since such disclosure there has been interest shown from time to time for the use of said catalytic liquid phase oxidation for the conversion of o-xylene to the anhydride of o-phthalic acid.
For example, U.S. Pat. No. 3,402,184 disclosed a continuous process for oxidizing o-xylene in an acetic acid solution containing ions of cobalt, manganese and bromine with air in three steps. Such o-xylene oxidation results in a solution of phthalic anhydride ("PAN") in the acetic acid. Said solution of PAN is combined with water and heated to convert the dissolved PAN to insoluble o-phthalic acid product.
British patent specification No. 856,245 published in 1960 discloses that a single step, neat (no extraneous solvent) oxidation of o-xylene does not go beyond 70 mole percent conversion of the xylene to phthalic anhydride and that a combination of fresh xylene and the 30 percent unoxidized materials cannot be oxidized. Then assuming that a single step oxidation of liquid o-xylene cannot exceed said 70 mole percent yield, said patent devices a two step oxidation wherein compounds having unoxidized alkyl groups still present in the first step reaction mixture are removed before such mixture goes to the second step, the remainder of the first step reaction mixture goes to the second oxidation step and the compounds removed from the first step mixture are returned as part of the feed thereto.
U.S. Pat. No. 3,920,735 discloses that the neat, single step oxidation of o-xylene with air in the presence of cobalt, manganese, zirconium and bromine produces a 60 mole percent yield of phthalic acid together with 0.97 mole percent yield of o-toluic acid, 2.95 mole percent yield of phthalide and 1.88 mole percent yield of 2-carboxybenzaldehyde. Such a PAN product is quite impure and contains a relatively large amount of difficultly removable phthalide.
Copending U.S. patent application Ser. No. 867,050, filed Jan. 5, 1978, describes an improved neat oxidation of o-xylene in the liquid phase to 85 to 92 mole percent yields of o-phthalic acid. The key to such substantially higher yield is to maintain the o-xylene oxidation product as the free dicarboxylic acid; that is, o-phthalic acid, and avoid conditions which permit such acid to dehydrate to its intramolecular anhydride, PAN. Such conditions are the conduct of the neat oxidation of liquid o-xylene with air at a temperature in the range of from 150.degree. C. up to 235.degree. C., at a gauge pressure of from 17 up to at least 28 kg/cm.sup.2 and in the presence of from 2 up to 7 weight percent water in the reaction mixture. Satisfactory rates of oxidation are under those conditions, obtained in the presence of catalysis provided on the basis of one gram mole of o-xylene by from 0.3 up to 10 milligram atoms of cobalt, from 0.15 up to 20 milligram atoms of manganese and from 0.225 to less than 60 milligram atoms of bromine. That is, the ratio of gram atom of bromine per gram atom total of cobalt and manganese is at least 0.5:1 but is less than 2:1. Also, when the cobalt concentration is less than 0.75 milligram atom per gram mole of o-xylene, then the difference between the actual cobalt concentration and the 0.75 milligram atom per gram mole of o-xylene can be supplied by zirconium on a gram atom for gram atom basis even though zirconium is not a polyvalent transition metal.
Said higher conversion and yield performing process is illustrated by and is limited to the use of batchwise or semi-continuous operations. By "semi-continuous" operation is meant the type of operation wherein a continuous mode of simultaneously charging air and o-xylene follows the first batchwise mode of charging all of the components of catalysis and water and heating the same to reaction temperature under reaction pressure, at the completion of charging all the xylene continuing only the charging of air until, for all practical purposes, the consumption of oxygen ceases and then stopping the air, and discharging all the reaction mixture for processing to recover the o-phthalic acid product. Heating to reaction temperature of the mixture of water and components of catalysis can be accomplished by heat of reaction released by oxidizing 5 to 25 percent of the o-xylene added with said mixture.
The simple batchwise and the semi-continuous operations of the above copending patent application both provide liquid reaction products containing 89 to 92 weight percent o-phthalic acid. Said simple batchwise operation produces o-phthalic acid in yields of up to 92 mole percent and consumes 5.5 mole percent of o-xylene by total combustion as determined by measurement of the total oxides of carbon produced. Said semi-continuous operation results in an 85 mole percent yield of o-phthalic acid and a 9.7 mole percent total combustion of o-xylene.
The results of said semi-continuous neat air oxidations of o-xylene demonstrate that operating conditions were found to overcome the yield limitation problem of single step oxidation mentioned in British Pat. No. 856,249. Also from the results of said semi-continuous neat air oxidation of o-xylene one would assume that its operating conditions were applicable for successful operation of truly continuous operation with perhaps the added disadvantage of a higher (above 9.7) mole percent total combustion of o-xylene.
However, such assumption, according to copending U.S. patent application Ser. No. 961,763, filed Nov. 17, 1978, was found not to be correct. Said assumption was in error because of the continuous addition of fresh xylene and components of catalysis with the requisite free water and the continuous removal of part of the reaction mixture, continuous operation produces a reaction mixture always containing unreacted xylene. Such difference is in contrast to the batchwise or semi-continuous operations whose reaction mixtures always have a diminishing o-xylene concentration before the reaction mixture is withdrawn from the site of reaction. Such difference in composition of reaction mixture is material because it gives rise to a problem interfering with successful continuous one-step oxidation not found during the development of the successful one-step operation by batchwise or semi-continuous operation.
Manifestation of said problem arising from the presence of unreacted o-xylene begins, we found, when the liquid reaction mixture contains 40 to 41 weight percent o-phthalic acid. Upon reaching such composition, there forms a liquid o-xylene phase and a liquid o-phthalic acid phase which contains the components of catalysis and the requisite 2 to 7 weight percent free water. Said two liquid phases, we found, were immiscible even though the reaction mixture is vigorously stirred. Because the components of catalysis are in the liquid o-phthalic acid phase, the catalysis is not effectively available for rapid oxidation of o-xylene and it accumulates as an increasing immiscible liquid phase. Up to reaching the 40 to 41 weight percent o-phthalic acid concentration, the oxidation reaction is vigorous but, as the immiscible liquid o-xylene accumulates, the oxidation reaction diminishes in vigor until the rate of oxidation becomes commercially unacceptable. Such vigor diminishing condition is readily observable from the volume ratio of o-xylene to water condensed from the exhaust from the oxidation zone. Such volume ratio is normally in the range of from 0.3:1.0 to 0.5:1.0, but the reaction's diminishing vigor is indicated by change of such ratio to 1:1 and finally to 2:1 for an unacceptable reaction rate.
Even when the o-phthalic acid concentration reached 40-41 weight percent in the batchwise and semi-continuous oxidation, there was, it is submitted, sufficient benzoic and o-toluic acid present in the reaction mixture to make miscible the o-xylene and the liquid o-phthalic acid solution of water and catalyst components.
Said U.S. copending application Ser. No. 961,763 filed Nov. 17, 1978 discloses a simple solution to said problem of forming two immiscible liquid phases. Said solution comprises the use of a miscibility aid (e.g., acetic acid or benzoic acid) which then provides a successful continuous single step neat oxidation of o-xylene with substantially complete conversion thereof and produces o-phthalic acid yields of 80 and higher mole percent but at the expense of excessive total combustion of o-xylene and coproduction of high ratio of phthalide to o-phthalic acid of less than 9 mole percent.
We have solved the aforementioned yield and conversion limiting formation of two immiscible liquid phases in a different manner by the use of two neat oxidation steps wherein o-xylene is fed to the first step and is therein converted to a liquid mixture which is directly oxidized per se in the second oxidation step. Our two-step oxidation differs from the two-step oxidation of British Pat. No. 867,050 because our two- step oxidation does not need a separation step to act upon the liquid effluent from the first oxidation step.
There is another prior art two-step air oxidation of a xylene which is disclosed in Canadian Patent No. 817,445. In general, the first step is a neat oxidation of a xylene and the liquid effluent from the first step is oxidized in the second step in the presence of up to 50 weight percent water. Our two step oxidation differs from that of the Canadian patent in that the second step of our process cannot tolerate water concentrations greater than between 7 and 8 weight percent.
According to British Pat. No. 856,245 mentioned before for its two-step neat oxidation, in the first step o-xylene is oxidized with air in the presence of from 0.5 up to 1000, preferably from 5 up to 1000 total miligram atoms of cobalt and manganese per mole of o-xylene with a gram atom ratio of manganese to cobalt of from 2:1 up to 9:1 and either no bromine or up to 15, preferably 4.5, milligram atoms of bromine per gram mole of o-xylene. Also, according to said British patent the feed to the second oxidation step should contain 1.0 weight percent or less of o-xylene. Further, the illustrative example of such two-step oxidation mentions conducting each step until oxygen consumption ceases, a condition indicative of batchwise, not continuous operation.
In our laboratories, we have conducted a first-step air oxidation of o-xylene in the absence of added reaction solvent, in the absence of bromine as a component of catalysis, in the presence of cobalt as the sole component of catalysis and cooling the spent air exhaust therefrom to condense byproduct water and unoxidized xylene without separation thereof for its recycle to the oxidation. By so doing with a cobalt concentration of from 0.5 to 1.0 milligram atoms per gram mole of o-xylene, the continuous air oxidation caused by unacceptably high burning of o-xylene end products as evidence by the total oxides of carbon production of from 0.05 up to 0.1 moles per mole of o-xylene charged. Even then the resulting reaction mixture did not have a xylene content of under 1.0 weight percent but, more undesirably, contained high boiling coproducts predominating in non-precursors of o-phthalic acid which adversely affected both the activity of catalysis and the course of the reaction in the second step conducted continuously.
We also found that the modification of such first continuous air oxidation of o-xylene by the addition of some manganese as a component of catalysis did diminish the co-production of the undesirable high boiling co-products but had little effect on the rate of o-xylene combustion to oxides of carbon. The only effective way we found to decrease the xylene burning rate was to employ some bromine in the continuous first step of air oxidation of liquid o-xylene. To decrease the burning of xylene there is used from 0.5 to 1.0 milligram atom of cobalt per gram mole of o-xylene charged to the continuous first step. This sharply decreased burning of o-xylene in the first step without the use of manganese therein, occurs provided there is also used a cobalt to bromine gram atom ratio of from 1:0.5 up to 1:10.
Such gram atom ratios provide from 0.25 up to 10 milligram atoms of bromine per gram mole of o-xylene charged to the continuous first step. However, such sharp reduction of production of total oxides of carbon still did not provide a reaction mixture of less than one weight percent o-xylene from the continuous first step air oxidation. Adjustment of conditions to lower the o-xylene content of the first step's reaction mixture produced in the presence of cobalt to bromine gram atom ratios of 1:0.5 to 1:10 caused an increase of formation in both total oxides of carbon and the undesirable high boilers which adversely affect catalytic activity and the course of reaction in the continuous second step.
From such experience, we concluded that British Pat. No. 865,245 did not disclose the essential guidance with respect to the successful conduct of a continuous solventless first step air oxidation of o-xylene to a reaction mixture which could be directly oxidized continuously with air to not only a high yield, 80 or more mole percent of o-phthalic acid, but also to a product of such a quality that its impurities were not of a nature to impede recovery of such acid as its anhydride in high yield and purity by commercially feasible techniques.
However, we did find conditions for the continuous conduct of the first step neat oxidation of liquid o-xylene with air to produce liquid reaction mixture which can be, without further processing, fed directly to a second step of continuous liquid phase neat air oxidation, which, when conducted under the conditions we found for the conduct of the second step, provides a high, 80 to 90 mole percent, yield of o-phthalic acid associated with impurities of a character and nature and in an amount which do not interfere with the recovery of the anhydride of phthalic acid in high yield and purity. The success of such a two-step process does not depend upon limiting the o-xylene content of the first step oxidation product to 1.0 weight percent or less as taught by British Pat. No. 856,245, but rather, surprisingly, in view of said patent, the first step liquid reaction mixture can contain up to but not more than 30 weight percent o-xylene and up to but not more than 40 weight percent o-phthalic acid.
Our continuous preparation of o-phthalic acid by two steps of catalytic air oxidation involving the neat oxidation of liquod o-xylene is more specifically described in the sections to follow.